The use of couplings and the like for forming pipe joints is well known. It is likewise well known to employ lock rings of sorts in association with the coupling in order to prevent in-service uncoupling of the joint by longitudinal pullout forces which might be imposed on the coupled pipe sections. Exemplifying the latter for use with metal piping is a coupling gasket construction disclosed in Miller U.S. Pat. No. 2,201,372.
Various arrangements and structures of pipe couplings are commercially available. A form of coupling particularly suited for underground gas piping is comprised of a cylindrical sleeve that is inwardly swaged in the course of compressing an annular gasket into sealing engagement about a contained pipe end. Such couplings are commercially available under the trademark POSI-HOLD and generally correspond to the structure disclosed in U.S. Pat. Nos. 3,429,587 and 3,915,480. Another form of swageable coupling likewise marketed under the same trademark is of a structure generally disclosed in Canadian Pat. No. 928,360.
Associated with the coupling construction disclosed in U.S. Pat. No. 3,915,480 is the mentioned pullout problem when coupling pipe of various compositions, most notably steel and plastic, and the government regulations relating to requirements for preventing pipe pullout. Department of Transportation Regulation No. 192.283(b) requires the joint to withstand tensile forces resulting from a temperature differential of 100.degree. F. As proposed in the patent, lock ring inserts contained within the gasket are utilized for the purpose of preventing pipe pullout with one form of insert being more suitable for plastic pipe and another form being more suitable for steel pipe. Operation of the lock ring in that construction is dependent on frictional restraint of the gasket elastomer in combination with an axially extending inclined plane formed on the inner surface of the coupling.
While the foregoing arrangement has worked well with a proven record of commercial success, the goal of meeting governmental standards for pipe pullout prevention with a more universal construction independent of the gasket and alternatively suitable for either plastic or metal pipe, has been long standing. Despite recognition of the problem, a solution therefor has not heretofore been known.